Still mystified.

Yesterday Granddad didn't go down to get the mail. The driveway had turned into a skating rink as the drizzle of rain froze in a flash. He knew the mail was down there, and he was really impatient for it. I didn't offer to slide down to get it because I was elbow-deep in butter, sugar and flour. (Yes, all afternoon. And evening, too.) When T came home Granddad wanted to know why he hadn't stopped for the mail. "If I stopped," said T, "I wouldn't have been able to get up the drive." But he did go down to bring up what was in the mailbox.

There was another mysterious package. It was firmly taped.
Granddad got it open. Inside was....chocolate! And another message:

He figured out that these keys were piano keys. But what does a leprechaun have to do with anything?

He found a locked box in the piano bench.

The key he found in his study exactly fit the lock.

Inside...another key and a mysterious message.

What can that mean?

It's a puzzle.